1. Field of the Invention
The present invention relates to an angiography method and apparatus wherein a two-dimensional array of pixels for display as an angiogram is determined by a projection or ray casting with respect to a volumetric data set obtained from signal samples of radiation exiting a region of a body under examination, which volumetric data set exhibits a contrast between flowing blood and stationary tissue. In its particular aspects, the present invention relates to angiography employing an integration projection (Ip).
2. Background of the Invention
In the medical imaging field, a three-dimensional image data set or volumetric image may be collected from X-ray Computed Tomography (CT), Magnetic Resonance Imaging (MRI), Positron Emitted Tomography (PET), Single Photon Emission Computed Tomography (SPECT), Nuclear Medicine Tomography and Ultrasound modalities. In addition, it is a common practice to collect a volumetric image as two-dimensional image data for each of a stack of relatively thin slices. However collected, a volumetric image has intensity values or voxels centered at lattice points in a generally regular 3D grid or array. Various of these modalities are useful to produce an angiogram from a collected volumetric image data set having intensity values exhibiting an enhanced contrast between flowing blood and stationary tissue, some by employing a preparatory step of injecting a contrast agent into the bloodstream.
Magnetic Resonance Angiography (MRA) is carried out by operating magnetic resonance imaging (MRI) apparatus to collect a three-dimensional data set of voxels using an RF and magnetic field gradient pulse sequence which produces an enhanced contrast between flowing blood and stationary tissue due to flow or velocity effects. Commonly used RF and magnetic field gradient sequences for angiography involve 2D time-of-flight or 2D or 3D phase-contrast methods. In the 2D time-of-flight method, a collection of spin resonance signals for multiple parallel slices is obtained. The flow sensitive contrast is due to substantially saturating stationary spins in the slice from which spin resonance signals are collected by relatively rapid repetition of close to 90.degree. flip angle slice selective RF excitation pulses so that only the unsaturated spins in blood flowing into the slice have relatively strong longitudinal magnetization just prior to experiencing the excitation pulses. This induces high intensity spin resonance signals from the inflowing blood, which intensity increases with the amount of inflow velocity component normal to the slice. A three-dimensional data set of voxel intensities is computed as the amplitudes obtained by two-dimensional Fourier transformation for each slice of samples of the spin resonance signals received during a read gradient for sequences repeated with different phase encoding gradient integrals. In the phase contrast method, bipolar gradient pulses are used to produce spin resonance signal phase dependence on velocity in the gradient direction. In the 3D method, a three-dimensional data set of spin resonance samples is collected. From these samples a three-dimensional data set of voxel intensities is obtained from phase values produced by a three-dimensional Fourier transformation. However obtained, the initial three-dimensional data set can be expanded by interpolating additional intervening layers or slices of voxels to produce a three-dimensional data set to be rendered for viewing purposes into a two-dimensional data set of pixels.
Display algorithms for processing three-dimensional Magnetic Resonance Angiography (MRA) data into two-dimensional form are known from H. Cline et al, "Volume Rendering and Connectivity Algorithms for MR Angiography" Magn. Res. Med. 18, pp. 384-394 (1991). A typical rendering method involves the formation of a projection image in a projection or viewing direction. Projection methods generally involve projecting or casting parallel rays through the three-dimensional data set in a viewing direction, the rays being in one-to-one association with the pixels of the projection image, and from the intensities of the voxels intercepted by or interpolated or resampled along the respective rays, determining the values of the associated pixels. Further, the two-dimensional pixel array determined by the projection rendering method can be expanded in size prior to viewing by interpolating additional intervening rows and/or columns of pixels. It is also useful to produce projection images from different angles of view corresponding to slowly rotating the viewing direction and to display these images sequentially in a cine loop. Such a cine helps the person reading the angiogram resolve vessel overlap and depth ambiguities.
The most widely used projection method is maximum intensity projection (MIP). With this method, which is computationally fast, the maximum intensities of voxels along or interpolated along ("interpolated" is herein meant to include analogous operations such as resampling) the respective rays are taken as the intensities of the associated pixels. MIP images discard much information that is present in the original volumetric image. A single MIP image does not provide any depth cues or vessel overlap information. Consequently, it is difficult to resolve depth ambiguities and vessel overlap even from a cine. Further, there is a loss of signal at vessel edges which causes apparent vessel narrowing and loss of vessel thickness information. Another deficiency of this method is that tortuous and looping vessels are difficult to distinguish from vascular aneurysms.
One alternative projection method is the so-called integration projection (IP) method in which the sums of the intensities of voxels along (or interpolated along) the respective rays are taken as the values of the associated pixels. Fast implementations can be achieved by using the relationship that a 2D integration projection image of a 3D data set in a projection direction is proportional to the inverse 2D Fourier transform of the 2D data set intercepted by (or interpolated along) a plane through the origin of the Fourier transform of the 3D data set whose normal is in the projection direction. The IP method preserves vessel thickness information because pixel intensity increases with increased vessel thickness in the projection direction. Unfortunately, there are also large contributions from integrated background voxels which obscure important vessel detail in the angiogram image. A known variation of the IP method is to sum only those voxels which have intensities above a given intensity threshold set by the operator. If the threshold is set too low, the contribution of background voxels still obscure vessel detail and result in a generally noisy image. Conversely, if the threshold is set too high, there is a loss of small vessel and edge information.
P. J. Keller et al., "SIMP: An Integrative Combination with MIP", Proceedings of the Annual Meeting of the society of Magnetic resonance in Medicine, (1991) p.201 discloses a combination of IP and MIP methods. A scaled integration projection is used for each ray intercepting at least two voxels that have intensities above a user-defined threshold. For rays not intercepting at least two such voxels, a maximum intensity projection is used. Too low a threshold results in an angiogram primarily having the characteristics of an IP image, while too high a threshold results in an angiogram primarily having the characteristics of a MIP image.
F. R. Korosec et al., "A Data Adaptive Reprojection Technique for MR Angiography", Proceedings of the Annual Meeting of the Society of Magnetic Resonance in Medicine", (1991), p.820, discloses another method that combines integration and maximum intensity projections. In this method, the original volumetric image data is compared with a first intensity threshold to define a vessel mask which is then blurred and converted to binary form. The blurred binary mask is applied to the original volumetric image data and thereafter a second, presumably higher, intensity threshold is applied to form processed volumetric image data. The final image is a weighted average of an integration projection image of the processed volumetric image data, after roll-off by a non-linear intensity transformation, and a maximum intensity projection of the original volumetric image data. This method appears critically dependant on the values set for the thresholds and further, due to the absence of depth cues, ambiguities remain as to the relative positions of overlapping vessels, which must be resolved from sequential projections at incremental angles. Further, setting the thresholds sufficiently high to suppress background may eliminate vessel detail in the integration projection contribution to the final image and such detail may be too faint, without integration along the direction of view, to be apparent in the maximum intensity projection contribution to the final image.
J. Listerud et al., "TRAP: Traced Ray by Array Processor", Proceedings of the Annual Meeting of the Society of Magnetic Resonance in Medicine, (1991) p.757, describes pseudo-surface rendering utilizing dynamic range compression and artificial lighting followed by maximum intensity projection. Artificial lighting of a range image makes structures closer to the viewer brighter than and appearing superimposed in front of those which are more distant. However, the maximum intensity projection of the range image destroys much of the depth information produced by the artificial lighting as well as suffering from the already described deficiencies of MIP.